Around the globe, the increasing need for bandwidth and the pursuit for realizing FTTH (Fiber to the Home) speedup the global fiber industry. At the same time, they have accelerated the development of the semiconductor laser industry in the field of optical communication. With the increasing popularity and the development of broadband network, the demands for bandwidth continue to grow surprisingly. For instance, the annual growth rate of the network users is over 50%, meantime the annual growth rate of household users reach 300%. Nowadays the most low-cost and the most effective way to expand the bandwidth is to use dense-wavelength-division-multiplexing (DWDM) optical transmission system, which drives the growth of demand for semiconductor lasers explosively. As the light source, semiconductor lasers are the key component in the fiber-optic system. Generally, single mode semiconductor lasers have been widely used in fiber optic communication, and their quality is a decisive factor for the transmission performance of optical communications. Usually, high quality semiconductor lasers perform well in single-longitude-mode characteristic, which has good mono-chromaticity without mode jump. In order to guarantee the single longitude mode operation dynamically, one of the most effective ways is to form a Bragg gratings in the semiconductor laser waveguide to select a wavelength longitude mode for lasing. Such type of laser with Bragg grating in the laser cavity is called distribute feedback (DFB) semiconductor laser. The feature of DFB laser is that the grating structure is distributed in the whole resonant cavity so that the light can be amplified during the feedback. Benefiting from the apparent wavelength selection of the DFB semiconductor laser's resonant cavity, the mono-chromaticity of this kind of laser is superior to other normal semiconductor lasers. Generally DFB semiconductor lasers are index-coupling induced by the periodic change in the reflecting index. When the reflection at the front and rare facets of the laser is zero, theoretical analysis shows that in this ideal condition, there are two degenerate modes with the same lowest resonant cavity loss that are symmetrical around the Bragg wavelength in index coupling DFB semiconductor laser.
Meanwhile there is only one mode with the lowest resonant cavity loss that is exactly at the Bragg wavelength in gain coupling DFB semiconductor laser. Therefore, there are two longitude modes in the index-coupling DFB lasers theoretically.
For the practical DFB semiconductor lasers, there are always some reflections in both facets of the laser. Not only is the reflectivity not equal to zero, but the reflection phase is also uncertain. This is due to the fact that in the practical fabrication of devices, the position of the facets in the grating periods is incontrollable. For the pure refractive index coupling DFB semiconductor lasers, in a considerable number of phases, the mode degeneracy is eliminated and single-longitude-mode operation is achieved. This was the method by which the single longitude mode operation was achieved in the earlier index-coupling DFB semiconductor lasers. But the random reflection phase leads to low single mode yield, which is about 20%˜50% when the facets have no anti-reflection coating. Usually, the coating on the facets influences the single mode yield. When one facet is coated with low reflection film and the other facet is coated with high reflection film, the single model yield reaches 50%. Lasers prepared using this method has a side mode suppression ratio (SMSR) of bigger than 40 dB when working statically; however, under high speed modulation, the SMSR is smaller than 20 dB, which cannot meet the requirement of high speed optical communications. A solution for such a problem is to introduce a quarter-wavelength (λ/4) phase shift in the center of the DFB grating to eliminate the mode degeneracy and realize single longitude model operation. The biggest advantage of this method is that true dynamic single longitude mode operation is realized by the giant gap between the fundamental mode and the high-order modes [S. Akiba, M. Usami and K. Utaka, “1.5-m λ/4-shifted InGaAsP/InP DFB lasers (1.5-m λ/4 phase-shift InGaAsP/InP DFB laser), J. Lightwave Technol. Vol. 5th, pp. 1564-1573, November 1987].
λ/4 DFB semiconductor lasers could be used as direct-modulated lasers. The biggest merit of direct-modulated DFB semiconductor lasers is that the dynamic single longitude mode is still kept under high speed modulation (2.5 Gbit/s˜10 Gbit/s), which is very suitable for the high speed short-distance fiber-optic communication system such as local area network (LAN). Currently, the commercial direct-modulated DFB semiconductor lasers are available for 2.5-Gbit/s over distances of up to a few hundred kilometers and the threshold current is about 5 mA. The 10 Gbit/s direct-modulated DFB semiconductor lasers are becoming the focus of research. For example, Japanese Corporation Mitsubishi in the year 2000 reported a direct-modulated DFB semiconductor laser that applied in 10 Gbit/s LAN. The operating wavelength of the laser was 1.3 μm and λ/4 phase-shifted DFB grating was used. By reducing the electrode area and the laser cavity length (cavity length is 200 m), the modulation bandwidth of the laser is enhanced. And high temperature performance is improved by increasing the index-coupling coefficient. In the range of 25° C.˜70° C., the modulation bandwidth is larger than 10 GHz, and the transmission distance is more than 20 km using standard single mode fibers.
In DFB semiconductor lasers, the quality of the Bragg grating plays a very crucial role, which directly determines the quality and performance of the lasers. Besides the λ/4 phase-shifted grating, Bragg grating with complex structure also improves the performance of DFB semiconductor lasers [S. Nilsson, T. Kjellberg, T. Klinga, R. Schatz, J. Wallin, K. Streubel, “Improved spectral characteristics of MQW-DFB lasers by incorporation of multiple phase-shifts”, J. Lightwave Technol. Vol. 13, pp. 434-441, March 1995; Nong Chen, Y. Nakano, K. Okamoto, K. Tada, G. I. Morthier, R. G. Baets, “Analysis, fabrication, and characterization of tunable DFB lasers with chirped gratings, IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, pp. 541-546, April 1997]. In the fabrication processes of DFB lasers, grating writing is a difficult but important process. The quality of the DFB grating is the decisive factor in the devices' performance. If there are some errors in practical grating fabrication, they cannot be finely adjusted or corrected. Furthermore, non-uniform gratings with complicated structure cannot be achieved using relatively simple and low-cost holographic exposure, but can be fabricated by electron-beam lithography or other complex techniques. The λ/4 phase shifted Bragg grating is non-uniform, so λ/4 phase shifted DFB semiconductor laser bears the problems of high cost, low product yield and complicated manufacture.
The fabrication of complex Bragg grating on the semiconductor waveguide becomes a key technique in the manufacture of high-performance DFB semiconductor lasers. In year 2002, a sampled Bragg grating (SBG) with chirp in the sampling period (CSP) was proposed in the Chinese patent “the sampled Bragg grating for dispersion compensation and polarization model dispersion with novel sampling structure” to obtain the desired equivalent chirp in the grating period (CGP) (Chinese patent, CN02103383.8, Jia Feng, Xiangfei Chen, et. al, Year 2002). The earliest literature to introduce equivalent chirp was the paper by Xiangfei Chen et. al, “Analytical expression of sampled Bragg gratings with chirp in the sampling period and its application in dispersion management design in a WDM system”, IEEE Photonics Technology Letters, 12, pp. 1013-1015, 2000. The characteristic feature of equivalent chirp technology is that the desired equivalent chirps can be obtained only using sub-micron precision. This special Bragg Grating is an SBG 502, which has many reflection peaks. As shown in FIG. 4, based on Fourier analysis, an SBG consists of multiple ghost gratings and each reflection peak corresponds to a ghost grating. The ghost gratings are characterized by Fourier orders (0th, +1/−1st, +2/−2nd, +3/−3rd, . . . ,). For example, the center reflection peak at the center Bragg wavelength 404 is related to the 0th order ghost grating. The ghost grating is also described using the term channel. The 0th ghost grating 404 can be described as 0th channel. The two reflection peaks or channels 403 and 405 correspond to the 1st order ghost grating and −1st order ghost grating, respectively. The 1st order channel is located on the left (shorter wavelength) of the center Bragg wavelength and the −1st order channel is located on the right (longer wavelength). The 1st, −1st or other order ghost grating can do work similarly with a conventional Bragg grating. That is to say, within the working band, when the filtering characteristic of the ghost gratings is same as that of conventional Bragg gratings, the ghost grating is the “same” as the corresponding conventional Bragg grating. Therefore, we can use 1st and −1st order ghost gratings 403 and 405 to replace conventional Bragg grating 302 as feedback element. Other order ghost gratings (Fourier orders=±2, ±3, . . . ,) can also be used to replace the conventional Bragg gratings. However, due to their low effective index coupling coefficients, the high-order (Fourier orders=±2, ±3, . . . ,) ghost gratings are usually neglected. The 0th order ghost grating is also not usable because equivalent chirp cannot occur within. Then 1st and −1st order ghost gratings are selected in practical applications for their larger effective index-coupling coefficients. No matter which order is chosen, only one ghost grating should be used to replace the conventional Bragg grating.
For convenience, such a ghost grating that is used to replace conventional Bragg grating is called equivalent grating. In operation band of ordinary grating, the equivalent grating can completely substitute the ordinary grating. The various functional capacities of equivalent grating are achieved by changing the distribution of sampling periods. For the reason that the sampling period is generally much larger than the grating period, the fabrication of the specific Bragg grating (equivalent grating) can be simplified greatly at a much lower cost. And all kinds of equivalent gratings with different optical response can be designed and fabricated easily without changing phase mask in the fabrication of fiber Bragg grating. For example, based on this technique, all kinds of complex equivalent chirps can be realized only using sub-micron precision, while such equivalent chirps perform almost identically with the true chirp in the grating period. Chirp in the grating means that the grating period is non-uniform, and the grating with chirp in the grating period is called chirped grating. The first order equivalent chirp, second order equivalent chirp and higher-order equivalent chirp may be achieved independently using equivalent chirp technology illustrated in the above.
Yitang Dai and Xiangfei Chen et al. have brought forward a novel concept of equivalent phase shift in the patent “the sampling fiber grating for en/decoding in DS-OCDMA system and its facture” (CN200410009546.X). The concept is also introduced in the paper of “Equivalent phase shift in a fiber Bragg grating achieved by changing the sampling period”, IEEE Photon. Tech. Lett., vol. 16, pp. 2284-2286, 2004. Furthermore, a new kind of technique has been proposed to design and fabricate any physically feasible equivalent grating with desired filtering in the Chinese patent of “a kind of fiber grating used to realize arbitrary desired optical response” (CN200410007530.5). This technique is a new technique combining reconstruction algorithm and equivalent chirp methods, which is called “reconstruction-equivalent chirp (REC) technology. The detailed description of REC technology can also be referred to the paper “Sampled Bragg grating with desired response in one channel by use of a reconstruction algorithm and equivalent chirp”, Opt. Lett., vol. 29, 1333-1335, 2004. The term “REC” was first brought up in the paper “Correction of the repeatable errors in the fabrication of sampled Bragg gratings”, OFC'2005, OME20, 2005. With REC technology, all kinds of physically realizable equivalent gratings with desired filtering characteristics can be designed and fabricated using ordinary sub-micron precision setups. The equivalent gratings can replace conventional Bragg gratings because in their operation bandwidth, they have the same optical response. Namely, conventional Bragg grating with complex optical response can be replaced by the corresponding equivalent grating for the same optical response.
The conventional grating in DFB semiconductor laser can be replaced by equivalent grating. Such an equivalent grating can be designed and fabricated using equivalent chirp technology and equivalent phase shift technology. More complicated equivalent grating can be designed and fabricated using REC technology. It should be mentioned that equivalent chirp and equivalent phase shift technologies are the special cases of the REC technology.
Semiconductor lasers are manufactured on a laser diode wafer. Many semiconductor lasers can be integrated on a wafer. When using REC technology to fabricate DFB lasers, the laser wavelengths of the DFB semiconductor lasers in the wafer can be determined by the corresponding equivalent gratings, namely, by the Bragg wavelengths of the equivalent gratings. The Bragg wavelengths of equivalent gratings are determined by the sampling periods of the DFB grating structures. Thus, based on REC technology, the laser wavelengths can be controlled by adjusting the sampling period of every DFB semiconductor laser on the wafer. The laser wavelength can be changed by more than 60 nm on a laser diode wafer.
The performance of DFB semiconductor lasers also varies with the material that is used in the fabrication of the DFB lasers. Usually, materials for DFB semiconductor lasers are III-V, II-VI and IV-VI compound semiconductors and so on.